Present power conversion apparatus normally comprises a plurality of discrete electrical components joined by electrical conductors. These components are normally classified for expository purposes and for design purposes into subcircuits identified as power modulation circuits, filter circuits, rectifier circuits, etc., each of which perform a distinct and separate function important to the overall circuit. The individual sizes of the individual electrical components such as cored magnetic components and electrolytic capacitors, in combination with spacing between components in the interest of good circuit design, result in a power supply whose linear and volumetric dimensions, in many instances, almost equals or sometimes exceeds the circuitry or system being powered. In this age of the printed-circuit-board-mounted modular circuits, the power supply, because of its size, is usually located a considerable distance from the circuit being powered. As a result of this size limitation, the bulk power supply concept is used in which a single power supply supplies power to a great number of circuits whether they are operative or not at any particular moment, rather than having a dedicated power supply for each individual circuit which can be turned off when the circuit is not operative.
If one wishes to dedicate power supplies for a particular circuit and have them located physically close thereto, the size of the power supply must be reduced by a considerable amount from that of the accepted sizes of the power supplies in the current state of the art.
A typical switching-type power conversion circuit operates by storing energy in various ones of its discrete capacitive and inductive components for one cycle length periods of time determined by the switching frequency. An increase in switching frequency reduces the storage time interval and the level of energy stored in storage components in any one particular cycle of operation. It is apparent that this increase in frequency permits both the physical and electrical sizes of magnetic and capacitive storage elements to be reduced for any particular power capacity.
Inasmuch as a significant increase in operating frequency of a converter permits a significant size reduction in the circuit components on the basis of energy storage per unit volume, the fact that the switching frequency of power converters has not increased dramatically is indicative of other constraints on the increase of operating frequencies. For example, the switching speed of bipolar semiconductor switching devices is limited by charge storage thereby limiting the benefits to be achieved from high frequency operation. This may, in part, be overcome by use of an existing power MOSFET switching transistor; however, its switching speed is limited by its device capacitances and the parasitic inductance of its lead wires.
Conventional passive components also present problems at high frequencies. At high frequencies, the parasitic inductance and resistance of a capacitor decrease its efficiency. The interwinding capacitance of inductors and the self-heating of the wire and core of the inductor also limit the switching frequency attainable. Individual circuit components generally include parasitic electrical parameters which become excessive at high frequencies and considerable design effort must be expended to compensate for them. It is likely that an increase in size will be required to minimize losses and that this increase will offset all benefits expected on the basis of energy storage considerations.
The circuit layouts have numerous stray capacitances and inductances which detract from power supply performance at high frequency. Capacitors exhibit parasitic inductance and resistances that lower the performance thereof below acceptable values at high frequencies. Power magnetics have undesirable winding capacity at high frequency, and both inductors and capacitances exhibit undesirable self-resonant characteristics. Because of these complicating factors due to undesirable parasitic effects the actual trade off of frequency to attain size reduction is unlikely to be linear or predictable for any particular conventional topology of power conversion circuits. The final result is that to achieve significant size reduction, the operating frequency must be so high that the problems alluded to hereinabove overwhelm the power supply designer thereby limiting the advent of the very high frequency power conversion circuit.
So despite the theoretical advantages of high frequency operation of power conversion circuits, these circuits have not been developed because of the many component and design problems related to operational difficulties at very high frequencies.
FCC regulations concerning the control of electrical radiation generated place further constraints on the development of high frequency power supplies since the necessary arrangements to shield or eliminate the electromagnetic radiation add to the cost of the circuit and limit its commercial viability.
In order to fully realize the benefits of high frequency power conversion circuits, the specific power conversion circuit must be capable of operating without radiating significant electromagnetic interference and without seriously suffering the deleterious effects of parasitics of the circuit components. It must also be properly sized to permit its location adjacent to the circuits being powered and must further be efficient in operation and economical to manufacture in order to be commercially viable.